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 A004104 Number of self-dual signed graphs with n nodes. Also number of self-complementary 2-multigraphs on n nodes. (Formerly M1649) 7
 1, 1, 2, 6, 20, 86, 662, 8120, 171526, 5909259, 348089533, 33883250874, 5476590066777, 1490141905609371, 666003784522738152, 509204473666338077658, 636051958071749028811326, 1375164117171886868027357906, 4844133410739656724629165903483, 29777568550007746192195431057341474 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS A 2-multigraph is similar to an ordinary graph except there are 0, 1 or 2 edges between any two nodes (self-loops are not allowed). Of a(1) through a(22) only a(3) = 2 is prime. - Jonathan Vos Post, Feb 19 2011 REFERENCES F. Harary and R. W. Robinson, Exposition of the enumeration of point-line-signed graphs, pp. 19 - 33 of Proc. Second Caribbean Conference Combinatorics and Computing (Bridgetown, 1977). Ed. R. C. Read and C. C. Cadogan. University of the West Indies, Cave Hill Campus, Barbados, 1977. vii+223 pp. R. W. Robinson, personal communication. R. W. Robinson, Numerical implementation of graph counting algorithms, AGRC Grant, Math. Dept., Univ. Newcastle, Australia, 1976. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS Andrew Howroyd, Table of n, a(n) for n = 1..50 (terms 1..22 from R. W. Robinson) Edward A. Bender and E. Rodney Canfield, Enumeration of connected invariant graphs, Journal of Combinatorial Theory, Series B 34.3 (1983): 268-278. See p. 273. Frank Harary, Edgar M. Palmer, Robert W. Robinson, Allen J. Schwenk, Enumeration of graphs with signed points and lines, J. Graph Theory 1 (1977), no. 4, 295-308. R. W. Robinson, Notes - "A Present for Neil Sloane" R. W. Robinson, Notes - computer printout PROG (PARI) permcount(v) = {my(m=1, s=0, k=0, t); for(i=1, #v, t=v[i]; k=if(i>1&&t==v[i-1], k+1, 1); m*=t*k; s+=t); s!/m} edges(v) = {sum(i=2, #v, sum(j=1, i-1, if(v[i]*v[j]%2==0, gcd(v[i], v[j])))) + sum(i=1, #v, if(v[i]%2==0, v[i]\4*2))} a(n) = {my(s=0); forpart(p=n, s+=permcount(p)*3^edges(p)); s/n!} \\ Andrew Howroyd, Sep 16 2018 CROSSREFS Cf. A004102, A052111, A052112, A052113. Sequence in context: A177480 A089179 A177483 * A304932 A293032 A241497 Adjacent sequences:  A004101 A004102 A004103 * A004105 A004106 A004107 KEYWORD nonn,nice AUTHOR EXTENSIONS More terms from Vladeta Jovovic, Jan 19 2000 a(18)-a(20) added by Andrew Howroyd, Sep 16 2018 STATUS approved

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Last modified January 22 18:48 EST 2019. Contains 319365 sequences. (Running on oeis4.)